Realization of a practical video-on-demand (VOD) system which can instantly provides program data in response to a demand of a receiver has been desired. A VOD system able to individually respond to demands made at all times, however, requires an extremely large sized data transmission apparatus which performs complex processing.
Accordingly, a near-video-on-demand (NVOD) system which can simplify the configuration of the data transmission apparatus while realizing a function similar to that of a VOD system has been proposed. An NVOD system is a system that transmits program data to a plurality of channels at predetermined time intervals. It selects the channels for the programs starting from the next time zone or time period in response to requests made in a certain time zone and then provides those programs.
Even in an NVOD system, however, it is necessary to simultaneously reproduce a plurality of series of program data to be transmitted to a plurality of channels, so there is a desire that the reproduction of the plurality of series of program data be carried out by a simpler apparatus and by a more efficient method.
As a fundamental method of generating a plurality of series of program data, there is the method of providing a separate reproducing apparatus for each of the channels and transmitting the data to the plurality of channels by these. Further, in recent years, hard disc devices, magneto-optic disc devices, and other randomly accessible recording apparatuses which have large capacities and can perform high speed access have been realized. An NVOD system which records video data etc. in these recording apparatuses and transmits the data to the plurality of channels in a time division manner is disclosed in Japanese Patent Application No. 5-328349 previously filed by the present applicant.
Further, the present applicant has proposed in Japanese Patent Application No. 6-137018 an NVOD system with a method of recording data wherein continuous regions are read so as to read the data and transmit the data to a plurality of channels at a speed as high as possible.
In such a data transmission apparatus, however, to perform high speed access and to facilitate the control as much as possible, the data has been encoded with a predetermined fixed data rate. As a result, the encoding at the fixed compression rate is carried out even with respect to data that can be encoded with a higher compression rate than the fixed data rate, so there is a problem that the programs cannot be efficiently stored. Further, with respect to data for which encoding at a fixed compression rate is difficult, conversely the encoding is forcibly carried out at that fixed compression rate, so there is a problem that the quality of the picture and sound when reconstructing the data is deteriorated.